U.S. Pat. No. 4,644,378 describes one type of light-emitting semiconductor device which produces radiation in the visible and near-infrared portion of the spectrum. This device has a light-emitting region formed by a layered structure with very thin active layers of a first semiconductor material located between and separated by very thin barrier layers of a second semiconductor material which has a larger energy gap than that of the first semiconductor material. These alternating very thin layers form a superlattice structure. In this device, the active layers are all of equal thickness having a thickness of n monolayers. The barrier layers are also of equal thickness having a thickness of m monolayers where n and m are integers between 2 and 7 with the value of n+m.ltoreq.12. The maximum thickness of 7 monolayers is that at which the active layers are still sufficiently thin to exhibit a strong quantum well effect, while the barrier layers are thin enough to produce a pronounced zone folding effect. The active layers in this device may be formed of GaAs with the barrier layers consisting of AlAs.
U.S. Pat. No. 4,630,083 describes another type of light-emitting semiconductor device with a laminated active layer consisting of five GaAs layers about 10 nm thick and four AlGaAs layers about 10 nm thick, where the GaAs layers are separated from each other by one of the AlGaAs layers. The AlGaAs layers have a larger band gap than the GaAs layers providing a quantum well structure i.e. a superlattice structure. The GaAs ultrathin layers service as quantum wells and the AlGaAs ultrathin layers serve as barriers. Both U.S. Pat. Nos. 4,644,378 and 4,630,083 deal with transitions between conduction and valence bands in superlattice structures.
Another type of device is described in Applied Physics Letter, Vol. 51 of Sep. 21, 1987 on page 934. This device is a semiconductor photodetector with a superlattice structure, in particular a 10 .mu.m radiation detector. This detector consists of 50 periods of 70 .ANG. GaAs quantum wells separated by 133 .ANG. Al.sub.0.36 Ga.sub.0.64 As undoped barriers. In this device, an electron in the ground state of the quantum well can be raised to the first excited state by photoexcitation resulting in an intersubband transition in that layer. Under proper bias this electron can tunnel out of that layer providing a photon generated current. Another 10 .mu.m infrared detector is described in Physical Review Letters, Vol. 59, No. 21, Nov. 23, 1987. This last-mentioned device provides a structure where coherent tunneling of photoexcited electrons through a double-barrier superlattice structure can occur. Each period in this laminated structure consists of a 72 .ANG. thicker well of GaAs (doped), a 39 .ANG. thinner barrier of undoped Al.sub.0.33 Ga.sub.0.67 As, a 18 .ANG. thinner well of undoped GaAs and a 154 .ANG. thicker barrier of undoped Al.sub.0.33 Ga.sub.0.67 As. The wider uncoupled wells have two states E.sub.1 =47 meV and E.sub.2 =183 meV in the energy-band diagram whereas the narrow well has only one state E'.sub.1 =174 meV. Under forward bias where E'.sub.1 is approximately aligned with E.sub.2 of the adjacent period, the coherent tunneling of a photoexcited electron from the wide well through the state E'.sub.1 of the narrow well and out of the barriers is possible. These two articles are directed to infrared detectors having a superlattice structure.